Telegraph system



TELEGRAPH S YS TEM Filed March 25, 1939 s Sheets-Sheet 1 INVENTOR 645597 .51 Viki/4M ATTORNEY Dec. 1941- I G. s. VERNAM 2,266,451

TELEGRAPH SYSTEM Filed March 25, 1939 s Sheets-Sheet 2 Dec. 16, 1941. s, VERNAM 2,266,451

TELEGRAPH SYSTEM Filed March 25, 1939 v s SheetsSheet'3 )NVENTOR 67/. 495??" a I VP/VAM ATTORN E! Patented Dec. 16, 1941 TELEGRAPH SYSTEM Gilbert S. Vernam, Hackensack, N. J assignor to Postal Telegraph-Cable Company (New York), New York, N. Y., a corporation of New York Application March 25, 1939, Serial No. 264,071 14 Claims. (01. 17852) My invention relates to telegraph systems and more particularly to telegraph systems in which a plurality of different transmitters and receivers may be operatively associated with a single telegraph channel in such a way that the time of the channel is divided among the transmitters and receivers in actual operation according to a predetermined plan, in accordance with the demands of the various transmitters.

It frequently occurs that telegraph service betweencustomers offices is desired, but that the single customers do not have sufiicientbusiness to oceupya full channell It is desirable in such cases to arrange the system so that several of the customers may be allotted time on a single channel, so that the totaltime of the channel and the cost may be divided among several customers. However, if this is accomplished by an arbitrary fixed division of time, each customer will have use of the channel for a fixed amount of time, although he may not be transmitting and other customers may be waiting. For this reason systems have'been devised in which'the time of utilization of a single channel isso arranged that if only one customer desires to transmit, he may avail himself of substantially full channel speed. If two desire to transmit at the same time only substantially half 'of the channel speed will be utilized by each customer, and if three only a third, and so forth.

In arrangements of this nature provisionmust be made for switching from one transmitter to the next and for simultaneously switching from one receiver to the next so that each transmitter will cooperate with its corresponding receiver.

In accordance with my invention a system is arranged so that a single main channel is allotted to each of a plurality of customers transmitters or sub-channels in turn for an interval sufficient for the? transmission of a plurality of character signals, after which a transfer signal is transmitted which serves to switch the main channel to the next sub-channel. If the next sub-channel is idle another transfer signal will be transmitted to switch the 'main channel to another sub-channel and if all the sub-channels are idle, transfer signals'will be transmitted continuously, and the main channel will be switched continuously to each sub-channel in turn until one channel starts to send. It is an object of my invention to provide a varioplex system of the type outlined above, which combines a simpleswitching arrangement and a simplified transmission and receiving system.

It is a further object ofmy inventionto provide" a combination of reversals'of various pulses inthe different sub-channels of a varioplex arrangement, so, in event the transmitter and receiver are out of step, they will be quickly brought into step. I

It is a still further object of my invention to utilize, as the transfer signal, a blank character signal so that the transfer signal will correspond to the normal "id1e signal transmitted when blank tape is in the transmitter or when the circuit is interrupted because of a movement of the tape controlled auto-stop lever.

It is a still further object of my invention to provide an arrangement-such that the time allotted to any one or more of the transmitters associated with the single channel may be varied to divide the time between the various subscribers in a manner more nearly "consistent with a particular service desired.

It is a still further object of my invention to provide a varioplex arrangement in which the received impulses are reversed in a manner to correspond with the reversal of signals at the transmitter.

It is a further object-of my invention to provide a receiver arrangement for varioplex system in which a singleflmultiplex to-start-stop translator is' used for translation of all of the signals from the various sub-channels.

It is a still further object of my invention to provide an arrangement for switching individual receiving loops to a start-stop translator for reception of signals.

Other objects and advantages of my invention will be apparent from the detailed description of a preferred circuit arrangement according to my invention, made in connection with the accompanying drawings, in which Fig. 1 illustrates a preferred arrangement of the transmitter,

Fig. 2 shows a preferred arrangement of a varioplex receiving circuitfor cooperation with the transmitter shown in Fig. 1, and

Fig. 3 illustrates a modified arrangement of a varioplex receiver suitable for use with the transmitter illustrated in Fig. 1.

As shown in the various figures of the drawings, only four sub-channels are provided. It is clear, however, that any number of channels may be used. With four channels in use, as illustrated, assuming. a multiplex channel speed of 50 Words per minute, the actual speed attained Percentage of channel time used for messages Average speed per channel Number of subchannels in use Percent Sending circuit Turning now to Fig. 1, the circuit shows four tape transmitters, designated I0I-I04. These may be standard multiplex tape transmitters'or any other suitable type of transmitter. The transmitters are arranged so as to have insulated bus bars I05 and conductive bus bars I01. Individual message tapes may be sent into these transmitters from any suitable source, e. g. key- :board perforators or from start-stop reperforators, the latter being operated over lines from printer keyboards at customers ofiices. The tape normally hangs down in a loop between the perforator and the transmitter and the auto stop lever I06 is lifted to open its contacts and stop the transmitter if the tape loop should tighten due to the perforator stopping. At the end of each message or group of messages the customer should transmit a number of letter shift or carriage return signals to allow the last actual message character to pass'through the transmitter before the loop tightens and stops the transmitter. matic attachment to the reperforator or to the customers printer to perforate these signals whenever the sending printer is switched off to insure this condition.

The contacts of the four transmitters are wired in parallel to the windings of five sending relays I'I0 to ll4, and the contacts of these relays are connected to plus and minus batteries and to the five segments I20 to I24 of one channel of a multiplex sending distributor. Character signal code combinations set up on any one of the tape transmitters will operate corresponding sending relays, causing the distributor brush I25 to transmit signals over duplex line 200 in the usual manner. The battery polarities normally applied to the fifth sending relay II4, are reversed with respect to the other four relays. A corresponding reversal is normally made in the connections of the fifth relay at the receiving station so that these signals are correctly received in spite of this reversal. This reversal assists in maintaining synchronism between the multiplex distributors while the circuit is idle, and is well known in the art. Reversing relaysl26, I21 are provided for reversing the polarity of the number 1 and number 3 signal impulses on certain of the subchannels, as will be described later. The arrangement is not limited to reversal of the first and third impulses, but any of the impulses or "any combination of them may be reversed, making it possible to distinguish as many as thirtytwo subchannels one from another, if desired. Furthermore, distinction between channels may be made by transfer of certain impulses, such as 1 and 5, i. e. send the impulses in the order 5, 2, 3, 4, 1, instead of reversing polarities. This reversal of various signal impulses distinguishes each of the channels one from another, so that messages may not be properly received except when the coordinated transmitter and receiver Other means may be used such as an autoare connected to the line, and furthermore aids in bringing the transmitters and receivers into step.

I provide two step-by-step rotary switches with the sending circuit. One of these, switch I30, shown as an eight point switch provided with three wipers I33, I'3I and I32, should properly have a number of contact points in each bank level equal to or a multiple of the number of sub-channels. This switch, at each step, switches the circuit from one sub-channel transmitter to the next. The other switch I60 is simply a counting device for counting the number of character signals to be transmitted from each transmitter before switching to the next transmitter. In the present case, if forty-nine characters are to be transmitted before switching to the succeeding transmitter, switch I60 should be a fifty point switch.

In the circuit as shown, it is assumed that there are tapes in each of the transmitters so that the auto-stop contacts controlled by levers I06 are closed. With contacts 1 and 2 of transmitter IOI against the left hand or marking bus bar I01 as shown (to'transmit the letter A) the first two sending relays IIO, II I, operate in a circuit from grounded battery, winding of relays IIO, III in parallel, transmitter contacts and bus bar of transmitter IOI, winding I4I of control relay I40, to ground, so that the control relay I40 also operates. The multiplex brush I25 transmits negative current to line when it crosses segments I20 and I2I, positive current when it crosses segments I22, I23, and negative again when it crosses segment I24, the fifth impulse being normally reversed, as previously explained.

When the local brush I crosses segment I5I, current will flow through the upper winding I42 and the upper make contact of control relay I40 to three circuits in parallel as follows:

1. Through contact of stop relay I45, lower make contact of control relay I40, second wiper I3I and bank contact of transmitter control switch I30, tape lever I06, upper winding of differential relay I46 associated with transmitter IOI, and through the transmitter stepping magnet to ground.

2. Through the break contacts and lower winding of diiferential relay I46 and magnet of transmitter 1 to ground.

3. Through the lower winding of stepping relay I to ground.

As the current through both windings of differential relay I46 are equal and in opposite directions, this relay does not operate, but the transmitter magnet operates to step the tape ahead to the next character signal. While the magnet is operated all the transmitter contacts are held against the right hand insulated bus bar I05 so that the sending relays are all released. Control relay I40 is held operated, however, by current to its upper winding I42, while the brush is crossing segments I5I. Stepping relay I55 locks itself through its front contact and the back contact of rotary magnet I 56.

When the brush I50 passes from segment I5I, the current in the lower winding of stepping relay I55 is out OK, but this relay will remain energized until rotary magnet I 56 has completely operated and opened its contact. The stepping relay I55 will then release and release rotary magnet I56, which will step the wiper of counting switch I60 ahead one step. At this time control relay I40 and the stepping magnet of transmitter IIJI will also release. If the character signal in the tape-is not a blankj? one or more of the tape transmitter contacts will be moved against the left hand bus bar I01 and the corresponding sending relays will operate, in preparation for transmitting the succeeding character signals, during the next revolution of the multiplex distributor brush. Control relay I40 will also operate in series with the sending relays and will remain operated until after the local brush has crossed segment I5I during this revolution.

These operations will be repeated with each successive revolution of the distributor until a blank signal is transmitted. This can occur in any of three ways. (1) By the occurrence of a blank signal in the tape, (2) By the supply of tape becoming reduced to a point where the tape lever contacts are opened or (3) By the counting switch wiper reaching the grounded bank contact to operate stop relay I45.

If a blank signal occurs in the perforated tape, the transmitter contacts will be held against the insulated bus bar I05 when the transmitter magnet releases. The sending relays IIO to II 4, and control relay I40 will therefore remain released. The multiplex sending brush I25 will transmit the blank signal over the line and when the local brush I50 crosses segment I5I it will close a circuit through the upper back contact of the relay I40, and the lower winding of stepping relay I5'I associated with transmitter control switch I30, to three circuits in parallel as follows:

1. Back contacts of stop relay I45 and control relay I40, the third transmitter control switch wiper I32 and its bank contact, tape lever I06 and upper winding of differential relay I41 and the stepping magnet of transmitter I02.

2. Through the lower winding and stepping magnet of transmitter IOI as previously traced.

3. Through the lower winding of stepping relay I55 of the counting switch.

It should be noted that the bank contacts associated with the third wiper I32 of the transmitter control switch I30, are so connected that when the second wiper I3I is connected to the tape lever of one transmitter, the wiper 'I32 is always connected to the tape lever of the next transmitter. The release of control relay I 40 therefore acts to switch the stepping circuit from one transmitter to the next, in the same manner as though the transmitter control switch I30has been advanced one step. This makes unnecessary the use of an extra multiplex distributor segment to step switch I30 at an earlier point in the cycle.

As only the lower winding of the differential relay I45 associated with the transmitter I is now energized, this relay will operate and lock itself through its own contacts, also locking the transmitter stepping magnet, to hold contacts of transmitter IOI against bus bar I05. Equal current now flows through both windings of differ ential relay I41 associated with transmitter I02, so that this relay will release and, after brush I50 passes over segment II, the transmitter magnet of transmitter I02 will release allowing certain of its contacts, depending upon the code combination punched in the tape, to move to its left hand bus bar I01 and control the sending relays. The control relay I40 now operates'and signals are transmitted from transmitter I02 in the same manner as previously described for transmitter IN. The stepping relay for transmitter control switch I30 was operated when brush I50 crossed segment I5I immediately after the transmission of the blank signal. This causes rotary magnet I58 of this switch to operate.

After brush I50-passes off segment I 5I and rotary magnet I58 has completely operate-d, stepping relay I5'I will release, also releasing rotary magnet I58 and advancing transmitter control switch wipers one step. I The stepping circuit through the lower front contact of relay I40 will now extend to the tape lever of transmitter I02 instead of that of transmitter IOI, so that the second transmitter will continue to operate as long as control I40 is operated.

If, instead of a blank signal occurring in the tape of transmitter IN, the tape lever contacts should be opened, the first of the threev parallel circuits which extend through upper winding of differential relay. I46 will be opened, and, when brush I50 crosses segment I5I the differential relay will operate and lock itself, as well as the transmitter magnet. This holds all of the contacts of transmitter IOI against the right hand bus bar I05, releasing the sending relays to send a blank signal and also releasing control relay I40. After the blank signal has been transmitted, the local brush I50 crosses segment I5I operating stepping relay I51 for the transmitter control switch and releasing the differential relay I4'I for transmitter I02 in the same manner as though the blank signal has been caused by a lack of perforations in the tape, as previously described. The transmitter control switch I30 will advance one step after the stepping relay releases If the tape lever contact is not opened and a blank signal does not occur in the tape, trans-- mitter IOI will continue to operate until the Wiper of counting switch I60 reaches its grounded bank contact. This switch should preferably have at least fifty bank contacts so as to permit transmission of 49 or more characters, although fewer are shown in the drawings. When the switch wiper reaches the grounded contact, stop relay I will be energized. When brush I crosses segment I5I during the next revolution, circuit vI previously traced through the upper winding of differential relay I46, will be open with the result that the differential relay I46 of transmitter IOI will operate and lock this transmitter. This will cause transmitter IOI to send a blank signal with the result that transmitter control switch I30 will advance one step, transferring the circuit to transmitter I02, in the same manner as though the tape lever contact for transmitter IOI had been opened as previously described. After the transmission of the blank signal the counting switch wiper of switch I will advance and release the stop relay I45.

The first wiper I33 of transmitter control switch I30, controls reversing relays I26, I21, which reverse the polarity of the first and third impulses, respectively. When the switch is in the position shown, transmitter IIJI is operating and both reversing relaysare released. When the switch advances onestep to cut-in transmitter 202, Wiper I33 energizes relay 523 over an obvious circuit to reverse the p'olarity'of the first selecting impulse. When control switch I30 advances another step to cut-in transmitter I03, relay I26 is released and relay I2! operated, thus reversing the third selecting impulse. When control switch I 30 advances to the fourth step, relays I26 and I2! are both operated in series thus reversing both the first and third selecting impulses.

It may be desired in some cases to arrange the equipment so that more time is allotted to certain sub-channels than to other sub-channels. For example, the most time might be allotted to the subscriber who contracts to send the greatest amount of trafiic. The amount of time allotted to each channel may be readily varied in accordance with my invention by changing certain connections of counting switch I60. One simple method of accomplishing this result is illustrated in Fig. 1. According to this arrangement an additional bank terminal of counting switch I60 is grounded through a switch I10 only when the transmitter control switch is connected to the circuit of transmitter ll. As a result, stop relay I45 will operate when the counting switch reaches this extra grounded contact after a comparatively few letters have been transmitted from transmitter lfll. This will switch the circuit to transmitter I02 which will continue to operate until the counting switch releases at its permanently grounded contact. As a result the time normally allotted to a single sub-channel has been divided between two sub-channels. It is clear that other methods of varying the time allotted may be readily devised in accordance with the principle illustrated in this single example.

Receiving circuit A preferred embodiment of receiving circuit in accordance with my invention is illustrated in Fig. 2. According to this arrangement incoming signals over line 200, operate the line relay 20I in accordance with the signals sent from the sendtype provided with segmented phase plates with brushes 204, 205 on one shaft and a segmented commutator 206 on a second shaft rotating two and one-half times the speed of the main brush arm shaft. The sixth pulse contacts are provided in a local arrangement controlled by brush Received signals operate line relay 20! and it in turn operates printer relay 202 during the intervals while the chopper brush represented by contacts 201 are connected to the segments as shown. The chopper brush reaches the edge of these segments at a time corresponding to the center of the received signal impulse and at this time opens the operating and closes the locking circuit for printer relay 202, so that the printer relay armature will be held against one of its contacts during the time that the chopper brush is on the second segment.

In accordance with my invention I utilize a multiplex to start-stop translator shown enclosed in the broken line and indicated by reference numeral 210, of the type disclosed in the application of Hallden, Serial No. 221,895, filed July 29, 1938, (now Patent No. 2,196,069, granted April 2, 1940, for Multiplex start-stop extension transsixth pulse interval, only in case a blank selection-is received, in which case none of the five receiving selecting magnets 2l3 to 2|! is operated. The translator has also been modified by bringing separate leads from the return side of the selecting magnets, so that they can be switched from the marking to the spacing contacts of printer relay 202 by contact of switching relays 23] to 234.

As shown, the first switching relay 23! is oper ated and when signals are received over the multiplex distributor they will be re-transmitted as start-stop signals to the first receiving loop 22l. The first four selecting magnets 2I3 to 2l6, which are connected with the first four receiving segments 243 to 246, respectively, have their return circuits connected to the marking contact of printer relay 202, so that these magnets will operate only when negative or marking signals are received over the multiplex line, at a time corresponding to the time when the receiving brush 204 crosses the corresponding selecting segment. The fifth selecting magnet 2 I! is connected to the spacing contact of the printer relay 202, so that it will operate only when a positive or spacing impulse is received when brush 204 crosses segment 241. It is obvious, therefore, that the signals from the first transmitter which are transmitted with the fifth impulse reversed, will be correctly received and correctly re-trans mitted in the receiving loop.

When a blank signal is received, none of the selecting magnets 2I3 to 2|! will be operated and the sixth pulse cut-out lever 2 l8 will prevent the sixth pulse magnet 2l9 from releasing the cam shaft, to operate the sending contacts, but will permit the sixth pulse magnet 2I9 to operate the contacts 2I2 which control stepping relay 250. Stepping relay 250 will therefore operate and lock itself through contacts of the magnet and cause rotary magnet 25I to operate. Rotary magnet 25! will release stepping relay 250 after contacts 2I2 have opened, and it in turn will release rotary magnet 25| advancing rotary switch 252 one step. This releases the first switching relay 23! and operates the second relay 232, transferring the sending contacts from the first loop 22! to the second receiving loop 222. Signals from the multiplex circuit will now be transmitted over the second loop. The upper contacts of the second switching relay 232 switch the selecting magnet 2l3 from the marking to the spacing contactof printer relay 202 so that this magnet will now operate on positive instead of negative impulses. As a corresponding change was made at the transmitting station by the operation of relay 126 it is obvious that signals from the second transmitter 202 will be correctly received and re-transmitted to the second receiving loop 222.

In a similar manner, when a blank signal is received while translator 2l0 is connected to loop 222, rotary switch 252 advances to operate the third switching relay 233 to connect translator 2l0 to the third loop 223. Relay 233 also reverses the return circuit from the third selecting magnet 2I5 so that the third magnet now operates only on positive impulses. As relay I21 is now operated at the sending station, signals for the third sub-channel will be correctly received.

When the rotary switch 252 advances to switch to the fourth loop 224, the fourth switching relay 234 operates to reverse both the first and third selecting magnets 213 and. H5. As both Sub-channel synchronising f If the rotary switches at the transmitting and" receiving stations should get out of step with each other for any reason, they will get into step automatically after a few letter signals have been transmitted For example, if the receiving circuit is connected to loop 222 while the transmitting circuit is still connected to transmitter IN, the signals received by the subscriber connected to loop 222, will be unreadable as the first selecting impulse of every character signal will be reversed. Now if aletter E signal which consists of one marking impulse followed by four spacing impulses should 'betransmitted this" would appear to be a blank signal at the receivingstation, asthe first selecting magnet is new connected to the spacing contact vofqthe printer relay 202.. Rotary switch 252 at thereceiving station will, therefore, advanceone step connecting translator 210 to the third loop 223. Asthe third selecting magnet, 215 is nowconnected with the spacing contact of printer relay 202, all the signals will be received with the third impulse reversed and will be unreadable to the subscriber. When a space signal consisting of two spacing impulses, one marking impulse and two spacing impulses is transmitted from the sending station, this will appear to be a blank signal at the receiving station and rotary switch 252 will now step .to the fourth receiving loop. The first and third magnets 2l3 and-215 are now both reversed so that when a letter S signal (1 marking, 1 spacing, l marking and 2 spacing impulses), is transmitted from thesending station this will appear to be a blank signal at the receiving station and will step rotary switch 252 again, connecting translator 210 to the first receiving loop 22L As the first transmitter, IBI is operating at the sending station and the receiving circuit is now connected tothe first receiving loop 22!, it is obvious that the system is now in step for correct operation and will normally remain instep. As .the character E, space and S all occur frequently in English text, it is obvious that the system will get itselfinto step promptly. I

If the entire system happens to be-idle due to all tapelever'contacts being open, a continuous succession of blank signals will be transmitted from the sendingcircuit, each in the code for the corresponding sub-channel "and; the transmitter control switch I30 will advance continuously making one step for each revolution of the multiplexdistributor. Under these conditions if the receiving equipment should be out ofstep -the rotary switch'at the receiving end will remain at rest'until a blank signal inthe proper code for the subchannel-to'which it is "connected is received from the transmitting" station, after which it will'step ahead keeping the receiving equipment in step" with that of the'sendng station.

Modified receiver A modified receiving circuit arrangement is illustrated in Fig. 3. According tothis circuit, signals incoming over line 200 are received in line relay 30! so. as to. operate printer relay 302 in the same manner as described above. :Similarly, thereceived signals are applied to a multiing its armature.

are all polar relays preferably of the kind known.

305'similar to the distributor and chopper ar--v rangement shown in Fig. 2. The chopperhas been illustrated'difierently in Fig. 3 from that: in Fig.2, but it is clear that the operation thereof .is the same, the separate segments being shown in asingle line instead of interlocked as, was shown in Fig. 2. p The chopper brushes while on the even numbered segments serve to ground the common return leads of receiving relays 3 to 35, so, that if the main brush 304 is on one'of the re-- ceiving segments, current will flow tothe upper operating winding of the corresponding receiv-;

ing relays in a direction depending upon which:

contacts of the printer relay 302 is now touch -v Receiving relays 3 to 3l5 as electro-polar, such as the type PEL relays manufactured by Automatic Electric Company-.- These relays have two sets of contacts, as'shown,

and when operated or released by a current im-;

them. This polarizing current replaces the permanent magnet ordinarily used as part of a polar relay. The fifth relay 3J5 is wound in the re-* verse direction with respect to relays 3| I to 3M, so that the reversed fifth impulse sent from the. sending circuit will be properly repeated.

The receiving relays obviously operate in re-.- sponse to signals received over the multiplex channel and will set up in their contacts the same code combinations as are set up in thev tape transmitters at the transmittingstations. The lower contacts of these relays control connections to the segments of a start-stop sending distributor 320 which .may be switched to any of four sub-channel loop circuits 32l to 324, to transmit corresponding message signals in startstop code to any of four customers station print-- ers.' The upper contacts of the receiving :relays serve to distinguish a blank signal from a message signal and switch'the sixth pulse contact 308 controlledby cam 309 from the start mag-.- net 33I of the start-stop distributor to the step.- ping magnet 332 of sixteen point rotary switch. 335, whenever a blank signal is received, 1:

As shown, a message character from transmit ter I0! is being received over the multiplex channel. Afte'r the brush crosses the segments of the receiving distributor, sixth pulse contacts3l8 close a circuit for operating start magnet 331 of start-stop distributor320 so that brush 333 cantraverse the segments of the transmitter and the character signal will be transmitted over sub-. channel loop 32!. It should be understood that one terminal of the loop is normally connected to battery and the other terminal extends; over a line to a standard start-stop printer similar to plex distributor 303 and a chopper arrangement that disclosed in connection with Fig. 2 and then groundto the opposite polarity of battery: If desired any of these loop circuits may be connected to the sending, armof a duplex circuit or carrier channel to extenda sub-channel .to any distance from a multiplex, terminal. The loop may also beconnected to a reperforator for sending it to a sub-channel of another multiplex circuit, thus forming a sub-channel repeater for interconnecting two, multiplex circuits. I

Signals received over .the multiplex channel will continue to .betrepeated as'start-stop signals into the loop and. printer of sub-channel 32l until a blank. signal is received. When this -occurs all receiving relays will be released when the sixth pulse contacts 308 close and the step-' pi'ng magnet 332 of rotary switch 335 will operate instead of the start magnet 33!. Switching relay 34! will release when the stepping magnet 332 operates, thus closing sub-channel 32! directly through the back contacts of switching relay 34!, instead of through brush 333 and stop segments of distributor 329. When stepping magnet 332 releases, the switch wipers will be advanced and switching relay 342 will be operated to connect sub-channel 322 through its makebefore break contacts to the start-stop distributor-32ll. Simultaneously, a circuit will be established for operating reversing relay 350 in series with relay 342 to reverse the operating winding of the receiving relay for the number one impulse, so it will respond to spacing instead of marking signals to correspond to the signal transmitted from transmitter I02. Itshould be noted that the blank signal is not transmitted to either sub-channel and the loop remains closed while the channels are being switched.

From the previous description of the sending arrangement it is evident that a blank signal is transmitted whenever the circuit is switched from one sub-channel transmitter to the next, and it is now evident that these blank signals cause the receiving equipment to be switched from one subchannel to the neXt so that, when once properly started the receiving sub-channels will remain in step with the sending sub-channels regardless of whether actual messages are sent for all the transmitters or not. A second reversing relay 35! is provided for reversing the winding'of the third receiving relay when the third sub-channel is in use, and connections are made to operate both relays 350 and 35! on sub-channel 4, so that at all times the signals are properly reversed at the receiver to correspond with the reversal at the transmitter. The receiving circuit shown in Fig. 3 can be readily brought into step by the occurrence of characters frequently found in various messages in the same manner as described in connection with Fig. 2. Moreover, in event that none of the sub-channels are transmitting, the stepping magnet'will not operate to advance from one receiving channel to another until such time as a correspondence between the channel is obtained, as explained more fully in connection with the description of Fig. 1.

Although I have disclosed certain preferred embodiments of my invention in connection with illustrative examples shown in the drawings, it should be understood that this description is not intended as a limitation or definition of the scope of my invention. What I consider to be my'invention is defined in the appended claims.

What I claim is:

1. A telegraph system comprising a plurality of transmitters for transmitting successive character signals each composed of a plurality of current impulses, a corresponding plurality of re' ceivers for receiving said signals, a single transmission channel, means for successively associating said transmitters and corresponding receiversto. said transmission channel for communication, and means whereby said receivers and said transmitters will be broughti'n to step, in cluding means for changing a characteristic of different character signals transmitted from different ones of said transmitters.

2. A telegraph system according to claim 1 in which said means for changing a characteristic of said signal character comprises means for re versing the polarity of different signal current impulses in each different transmitter.

3. A telegraph system according to claim 1, further comprising means responsive to transmission of a predetermined number of characters from one transmitter for causing said successive association.

4. A telegraph system comprising a plurality of transmitters for transmitting successive character signals each composed of a plurality of impulses, a corresponding plurality of receivers for receiving said signals, a single transmission channel, means responsive to the transmission of a transfer signal for successively associating said transmitters and corresponding receivers with said transmission channel for communication, and

means whereby said receivers and said transmit-' ters will be brought into step with each other in the event they are not in proper step, said means including means for reversing different current impulses of said character signals transmitted from different ones of said transmitters.

5. A telegraph transmitter system comprising a plurality of transmitters for transmitting successive character signals each composed of a predetermined number of current impulses to corresponding receivers, a single transmission channel, means for successively associating said transmitters with said transmission channel, means for transmitting a transfer signal upon operation of said last named means, and connections for reversing different ones of said current impulses from each transmitter, whereby signals received on a receiver other than the corresponding receivers are not intelligible.

6. A telegraph transmitter system comprising a plurality of transmitters for transmitting successive character signals each composed of a predetermined number of current impulses to corresponding receivers, a single transmission channel, means responsive to transmission of a transfer signal for successively associating said transmitters with said transmission channel, and connections for reversing different ones of said current impulses from each transmitter, whereby signals received on a receiver other than the corresponding receiver are not intelligible.

7. A telegraph transmitter according to claim 6 wherein said connection for reversing said ourrent impulses comprises means for reversing the '3 means responsive to receipt of said transfer signal for connecting the succeeding receiver to said channel, and means whereby the bringing into step of a transmitter and the corresponding receiver is facilitated, including means for reversing difierent current impulses in each of said transmitters so that said transfer signal in any one of said channels corresponds to a frequently occurring character in each of said other channels.

9. A telegraph system according to claim 8, wherein said multi-unit code is the standard five-unit code, said transfer signal comprises a blank signal, and four transmitters are provided, the character signal elements of one of said transmitters being transmitted witnno reversals,

the second transmitter having the first impulse reversed, the third transmitter having the third impulse reversed and the fourth transmitter having the first and third impulses reversed, whereby said blank signal in one channel represents the letter E, the word space signal and th letter S, respectively, in the remaining channels.

10. A telegraph system according to claim 8, further comprising means responsive to transmission of a predetermined number of characters from, one of said transmitters for causing transmission of said transfer signal, and means operatively associated with at least one of said transmitters for varying the said predetermined number of characters.

11. A telegraph receiver system comprising a plurality of telegraph receivers, means for successively operatively connecting said receivers to a single channel for receiving multiple-impulse telegraph signals, means responsive to a transfer signal for operating said successively operating connecting means, and means operative upon connection of each receiver for producing a reversal of predetermined ones of said received signal impulses.

12. A telegraph transmitting system comprising a plurality of transmitters, a single trans mission channel, and means for successively operatively associating said transmitters with said channel comprising a first rotary switch means, a circuit including said first rotary switch means for completing said operative association, a second rotary switch means, normally'incomplete circuit connections from said secondary'rotary switch means to the succeeding transmitters, means responsive to transmission of a transfer signal for opening said first named circuit and completing said normally incomplete circuit, and I means for subsequently advancing said first and second rotary switch means to complete a circuit for said succeeding transmitter over said first rotary switch means and to form a normally incomplete circuit for the next succeeding transmitter.

13. A telegraph receiver system comprising a plurality of transmitters, a plurality of receivers, a single transmission channel, means for successively connecting said receivers to said single channel for receiving multiple impulse telegraph signals, a single translator for transferring said signals successively from said channel to said receivers, means for altering differently the character of signals transmitted from each transmitter, and means operatively associated with the corresponding receiver loop for returning signals so altered to unaltered form whereby signals from each transmitter will be correctly received in the corresponding receiver loop.

14. A telegraph system comprising a single transmission channel, a plurality of transmitters for transmitting to said channel character signals each composed of a combination of a plurality of impulses, means intermediate .said channel and said transmitters for modifying the character impulse combinations sent by each transmitter to said channel, means responsive to a particular character signal sent from any of said transmitters to said intermediate means for associating the next successive transmitter with said channel, a plurality of receivers and means under control of said particular one of said character signals as modified by said intermediate means for associating proper ones of said receivers successively with the transmission channel.

GILBERT S. VERNAM. 

